ESD protection with increased current capability

ABSTRACT

A stackable electrostatic discharge (ESD) protection clamp ( 21 ) for protecting a circuit core ( 24 ) comprises, a bipolar transistor ( 56, 58 ) having a base region ( 74, 51, 52, 85 ) with a base contact ( 77 ) therein and an emitter ( 78 ) spaced a lateral distance Lbe from the base contact ( 77 ), and a collector ( 80, 86, 762 ) proximate the base region ( 74, 51, 52, 85 ). The base region ( 74, 51, 52, 85 ) comprises a first portion ( 51 ) including the base contact ( 77 ) and emitter ( 78 ), and a second portion ( 52 ) with a lateral boundary ( 752 ) separated from the collector ( 86, 762 ) by a breakdown region ( 84 ) whose width D controls the clamp trigger voltage, the second portion ( 52 ) lying between the first portion ( 51 ) and the boundary ( 752 ). The damage-onset threshold current It 2  of the ESD clamp ( 21 ) is improved by increasing the parasitic resistance Rbe of the emitter-base region ( 74, 51, 52, 85 ), by for example, increasing Lbe or decreasing the relative doping density of the first portion ( 51 ) or a combination thereof.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 12/689,666, filedJan. 19, 2010.

FIELD OF THE INVENTION

The present invention generally relates to semiconductor devices andmethods for fabricating semiconductor devices, and more particularlyrelates to semiconductor devices used for electrostatic discharge (ESD)protection in integrated and other circuits.

BACKGROUND OF THE INVENTION

Modern integrated circuits (ICs) and the devices therein are at risk ofdamage due to electrostatic discharge (ESD) events. This is well knownin the art. Accordingly, it is commonplace to provide an ESD clamp(voltage limiting device) across the input and/or other terminals ofsuch devices and IC's. FIG. 1 is a simplified schematic diagram ofcircuit 20 wherein ESD clamp 21 is placed, for example, betweeninput-output (I/O) terminal 22 and ground or common terminal 23 of an ICto protect other devices on the chip, that is, to protect “circuit core”24 also coupled to I/O and common terminals 22, 23. Person of skill inthe art will understand that ESD clamp 21 may be placed across anyterminals of the IC or other device or circuit, and reference herein toI/O terminals is intended to include any and all other terminals notmerely those used for input or output signals. Further, the Zener diodeillustrated in block 21 of FIG. 1 is merely for convenience ofidentifying the voltage limiting function of ESD block 21 and notintended to imply that a Zener diode is present therein.

FIG. 2 is a simplified schematic diagram illustrating internalcomponents of ESD clamp 21 utilizing bipolar transistor 25, havingemitter 26, collector 27, base 28, and internal resistance 29. When thevoltage across terminals 22, 23 rises beyond a predetermined limit,bipolar transistor 25 turns on, limiting the voltage across terminals22, 23, desirably to a level below that capable of damaging circuit core24.

FIG. 3 shows simplified plot 30 of transmission line pulse current (I)versus voltage (V) for a typical electrostatic discharge (ESD)protection device such as, for example, the device of FIG. 2. As theapplied voltage is increased, very little current flows until triggeringvoltage 31 is reached at voltage Vt1. Once triggered into operation, theESD device conducts and the current increases to holding point 32 withcurrent Ih and voltage Vh. Depending upon the internal impedance of thevoltage source, current and voltage may further increase to point 33 atcurrent It2 and voltage Vt2, beyond which destructive failure may occurleading to further current increase accompanied by voltage decrease. It2is also referred to as the damage onset current threshold.

Electrostatic discharge (ESD) protection devices are intended to remainquiescent during normal operation of the associated semiconductor (SC)device(s) or non-SC device(s) or integrated circuit (IC) (i.e., theprotected element(s) of circuit core 24) having a normal operatingvoltage Vo, but turn on when excessive voltage arises, therebypreventing damage to the protected element(s). The triggering voltageVt1 of the ESD device should exceed the maximum normal DC operatingvoltage Vo(MAX) of the protected elements; otherwise the ESD device willinterfere with normal operation of the protected elements. Further, Vt1should be less than, for example, a voltage V_(TR) (usually a transientvoltage) large enough to damage the protected element(s), hereafterreferred to as the protected element break-down voltage, abbreviated asV_(TR)(PEBD). Thus, the ESD device should be designed so thatVo(MAX)<Vt1<V_(TR)(PEBD).

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 is a simplified schematic diagram of a circuit wherein an ESDclamp is placed between an input-output (I/O) terminal and a ground orcommon terminal of an IC or other circuit to protect other devices onthe chip, that is, the “circuit core” coupled to the I/O terminals;

FIG. 2 is a simplified schematic diagram illustrating the internalcomponents of the ESD clamp of FIG. 1;

FIG. 3 is a simplified plot of current versus voltage for anelectrostatic discharge (ESD) protection device, such as the device ofFIG. 2;

FIG. 4 shows a simplified cross-sectional view of an ESD clampimplemented in a semiconductor substrate and suitable for use in thecircuits of FIGS. 1-2, according to an embodiment of the presentinvention;

FIG. 5 is a simplified plot of DC breakdown voltage in volts as afunction of lateral base-collector spacing dimension D in micrometersfor the device of FIG. 4 over an extended range of spacing dimensions D;

FIGS. 6-7 are simplified schematic diagrams of serially cascaded (e.g.,stacked) ESD transistors of the type illustrated in FIG. 4 but withspacing dimensions D limited to zone Z1 of FIG. 5, wherein FIG. 6 showsa 2-stack and FIG. 7 shows a 3-stack;

FIG. 8 is a simplified plot of DC breakdown voltage in volts as afunction of lateral base-collector spacing dimension D in micrometersfor different numbers of serially cascaded (stacked) ESD transistors ofthe type illustrated in FIG. 4 with spacing dimension D limited to zoneZ1 of FIG. 5;

FIG. 9 shows a simplified cross-sectional view of a 2-Stack ESD clampimplemented in a semiconductor substrate according to a furtherembodiment of the present invention;

FIGS. 10-15 show simplified schematic diagrams of serially cascadedstacks of ESD transistors, wherein FIGS. 10-12 show 2-stack combinationsand FIGS. 13-15 show 3-stack combinations, according to still furtherembodiments of the present invention;

FIG. 16 is a simplified plot of DC breakdown voltage in volts as afunction of lateral base-collector spacing dimension D in micrometersfor different 2-stacks of cascaded ESD transistors, comparing theresults for 2-stacks having different dimensions D within the stackaccording to yet further embodiments of the present invention;

FIG. 17 shows a simplified cross-sectional view of an ESD clampimplemented in a semiconductor substrate and analogous to that of FIG.4, but according to a yet further embodiment of the present inventionwherein a bidirectional ESD function is provided;

FIGS. 18-22 are simplified cross-sectional views of an ESD clamp of thetype illustrated in FIG. 9 during various stages 118-122 of manufactureaccording to still yet further embodiments of the present invention andshowing additional detail;

FIGS. 22-1 and 22-2 are simplified cross-sectional views of an ESD clampof the type illustrated in FIG. 9 showing variations of manufacturingstage 122 of FIG. 22, according to other embodiments of the presentinvention and showing additional detail;

FIGS. 23-28 are simplified cross-sectional views of an ESD clamp of thetype illustrated in FIG. 9 during various stages 123-128 of manufactureaccording to still other embodiments of the present invention andshowing additional detail; and

FIGS. 29-30 are simplified cross-sectional view of ESD clamps, analogousto those of FIG. 4 and suitable for use in the circuits of FIGS. 1-2,but according to yet still other embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, or the following detailed description.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the invention. Additionally, elements in thedrawings figures are not necessarily drawn to scale. For example, thedimensions of some of the elements or regions in the figures may beexaggerated relative to other elements or regions to help improveunderstanding of embodiments of the invention.

The terms “first,” “second,” “third,” “fourth” and the like in thedescription and the claims, if any, may be used for distinguishingbetween similar elements and not necessarily for describing a particularsequential or chronological order. It is to be understood that the termsso used are interchangeable under appropriate circumstances such thatthe embodiments of the invention described herein are, for example,capable of operation in sequences other than those illustrated orotherwise described herein. Furthermore, the terms “comprise,”“include,” “have” and any variations thereof, are intended to covernon-exclusive inclusions, such that a process, method, article, orapparatus that comprises a list of elements is not necessarily limitedto those elements, but may include other elements not expressly listedor inherent to such process, method, article, or apparatus. The term“coupled,” as used herein, is defined as directly or indirectlyconnected in an electrical or non-electrical manner. As used herein theterms “substantial” and “substantially” mean sufficient to accomplishthe stated purpose in a practical manner and that minor imperfections,if any, are not significant for the stated purpose.

As used herein, the term “semiconductor” is intended to include anysemiconductor whether single crystal, poly-crystalline or amorphous andto include type IV semiconductors, non-type IV semiconductors, compoundsemiconductors as well as organic and inorganic semiconductors. Further,the terms “substrate” and “semiconductor substrate” are intended toinclude single crystal structures, polycrystalline structures, amorphousstructures, thin film structures, layered structures as for example andnot intended to be limiting, semiconductor-on-insulator (SOI)structures, and combinations thereof The term “semiconductor” isabbreviated as “SC.” For convenience of explanation and not intended tobe limiting, semiconductor devices and methods of fabrication aredescribed herein for silicon semiconductors but persons of skill in theart will understand that other semiconductor materials may also be used.Additionally, various device types and/or doped SC regions may beidentified as being of N type or P type, but this is merely forconvenience of description and not intended to be limiting, and suchidentification may be replaced by the more general description of beingof a “first conductivity type” or a “second, opposite conductivity type”where the first type may be either N or P type and the second type thenis either P or N type.

In order to be able to build ESD devices that have different Vt1 valuesto meet the protection needs of different core circuits 24 (see FIG. 1),it is common to design ESD devices such that Vt1 depends upon thespacing of a particular device dimension. For example, bipolartransistor 25 of FIG. 2 is often a lateral transistor wherein Vt1depends upon a base-collector spacing dimension D. Transistor 40 of FIG.4 (described below) illustrates a lateral transistor havingbase-collector spacing dimension D. One of the difficulties of usingbipolar transistors such as bipolar transistor 25, 40 in ESDapplications is that there can be significant variation ΔD inbase-collector spacing dimension D across a SC wafer and/or SC die as afunction, for example, of the azimuthal orientation of transistor 25, 40on the wafer or die. In addition, the spacing dimension D can havesignificant variation ΔD from one wafer to another, e.g. betweenmanufacturing different lots. This has the result that Vt1 of nominallyidentical devices can be different in different regions of the same ICand from manufacturing lot to manufacturing lot, depending, for example,on their relative azimuthal orientation on the IC die or wafer. This Vt1variation can adversely affect overall manufacturing yield and is notdesirable. Various process modifications may be used to minimize sucheffect, but such modifications are often accompanied by an undesirableincrease in manufacturing cost or other difficulties. The Vt1 variationcan become especially acute when such ESD clamp transistors arecascaded, that is, serially coupled in stacks in order to obtain highervales of Vt1 than can be provided by single ESD clamp transistor 25, 40.

There is an ongoing need to provide improved ESD clamps that operate atmore consistent trigger voltages Vt1 independent of their location ororientation on a particular IC, especially stacks of ESD clamps adaptedto provide higher values of Vt1 than can be obtained with single ESDtransistor 25. Additionally, it is desirable that the current handlingcapabilities of the ESD clamps, for example the damage onset currentthreshold It2, be made as large as possible for a given ESD device areaand trigger voltage. Further, it is desirable that the improved ESDclamps be obtainable without significant modification of themanufacturing process used for forming the clamps and the associatedcircuit core of the IC. Furthermore, other desirable features andcharacteristics of the present invention will become apparent from thisdetailed description of the invention and the appended claims herein,taken in conjunction with the accompanying drawings and the backgroundof the invention.

FIG. 4 shows a simplified cross-sectional view of ESD clamp transistor40, 70 implemented in semiconductor substrate 72 according to anembodiment of the present invention. Transistor 40, 70 fulfills thefunction of transistor 25 in FIG. 2 and ESD clamp 21 in FIG. 1.Transistor 40, 70 is formed in substrate 72 (e.g. P) having uppersurface 71 and with N type buried layer (NBL) region 73 therein.Overlying NBL 73 is region 74 (e.g., P) extending from NBL 73 to surface71 and within which are formed shallow trench isolation (STI) regions79, deep trench isolation (DTI) regions 792, N WELL regions 761, 762(collectively 76) with contact region 80 (e.g., N+) and P WELL region75. Doped contact region 77 (e.g., P+) is provided in P WELL region 75to make Ohmic contact to P WELL region 75. P WELL region 75 is generallysomewhat more heavily doped than P region 74. Doped region 78 (e.g., N+)in P WELL region 75 serves as the emitter, P WELL region 75 (withportion 85 of P region 74) serves as the base, and N WELL region 86, 762with N+ contact region 80 serves as the collector of transistor 40, 70.Doped region 77 (e.g., the base contact region) and emitter 78 areseparated by lateral distance 50, also referred to as lateralbase-emitter (Lbe) spacing 50. Dielectric layer 81 is convenientlyprovided on surface 71 with openings therein extending to base contactregion 77, emitter 78 and collector contact region 80. Conductor 82makes Ohmic contact to collector contact region 80, and conductor 83makes Ohmic contact to base contact region 77 and emitter 78, connectingregions 77, 78 together. Conductor 82 of transistor 40, 70 isconveniently coupled to terminal 22 and conductor 83 of transistor 40,70 is conveniently coupled to terminal 23 of ESD circuit 20. Further Nregion 86 is provided in Ohmic contact with N WELL region 762.Base-collector spacing dimension D is determined by the lateralseparation between boundary 752 of P Well (e.g., base) region 75 andboundary 929 of further N region 86, wherein avalanche breakdown region84 spans portion 85 of P region 74 between boundary 752 of P WELL baseregion 75 and boundary 929 of further N region 86. The relative dopingof the various regions is discussed in more detail later. The conventionis generally followed hereafter wherein reference number 40 is used toidentify transistors that have base-collector spacing dimensions Dlimited to central zone Z1 of FIG. 5 and reference number 70 is used torefer to transistors that have base-collector spacing dimensions D fromany of zones Z1, Z2, Z3 of FIG. 5.

FIG. 5 shows simplified plot 88 of AC breakdown voltage Vt1 _(AC) (trace88-1) and DC breakdown voltage Vt1 _(DC) (trace 88-2) in volts as afunction of lateral base-collector spacing dimension D in micrometersfor device 40, 70 of FIG. 4 over an extended spacing range, in thisexample, from D less than about 0.7 micrometers to D of about 3.5micrometers. It will be noted that the values of Vt1 _(AC) (trace 88-1)and Vt1 _(DC) (trace 88-2) are close together and have substantiallyidentical slopes (ΔVt1/ΔD). Plot 88 of FIG. 5 may be divided into threezones: (1) central Zone-Z1, (2) leftward peripheral Zone-Z2, and (3)rightward peripheral Zone-Z3. Zone Z1 has spacing dimensions D_(Z1)corresponding to D_(A)<D_(Z1)<D_(B), zone Z2 has spacing dimensionsD_(Z2) corresponding to D_(Z2)≦D_(A) and zone Z3 has spacing dimensionsD_(Z3) corresponding to D_(Z3)≧D_(B). In this example D_(A) is about1.2-1.3 micrometers and D_(B) is about 2.4-2.5 micrometers, butdifferent values may be obtained in other embodiments. Central zone Z1corresponds to trace 61 for single ESD-Z1 device 40 of FIG. 8. Vt1varies approximately linearly with spacing dimension D_(Z1) in centralzone Z1 with slope (ΔVt1/ΔD)_(Z1) (for both AC and DC) of about 25 voltsper micrometer. It will be noted that slope (ΔVt1/ΔD)_(Z2) of Vt1 versusD in zone Z2, and also slope (ΔVt1/ΔD)_(Z3), of Vt1 versus D in zone Z3are significantly smaller than slope (ΔVt1/ΔD)_(Z1) in zone Z1. Forexample, slope (ΔVt1/ΔD)_(Z2) has a value of about 0 to 3 volts permicrometers and slope (ΔVt1/ΔD)_(Z3) has a value of about 0 to 9 voltsper micrometer, as compared with the above-noted value for slope(ΔVt1/ΔD)_(Z1) of about 25 volts per micrometer. Stated another way, theslope of region Z1 is at least 2 or more times greater than the slope ofregions Z2 or Z3. It has been found that these differences in Vt1 versusD sensitivity among Zones Z1 to Z3 can be used to provide stacked ESDdevices with reduced sensitivity to variations ΔD in spacing dimensionD.

FIGS. 6-7 are simplified schematic diagrams of ESD clamps 65, 66comprising serially cascaded (e.g., stacked) ESD transistors 40 of FIG.4 all with spacing dimension D from zone Z1 of FIG. 5. FIG. 6 shows2-stack clamp 65 and FIG. 7 shows 3-stack clamp 66. Stated another way,2-stack ESD clamp 65 comprises two series coupled devices or transistors21, 25 of FIGS. 1-2, wherein terminal 22 of the first transistor iscoupled to terminal 23 of the second transistor, and terminal 23 of thefirst transistor and terminal 22 of the serially cascaded secondtransistor are coupled across core circuit 24, wherein transistors 40fulfill the function of devices or transistors 21, 25 with the spacingdimensions D as noted above. Analogously, 3-stack clamp 66 comprisesthree serially cascaded devices 40 coupled across circuit core 24. FIG.8 is a simplified plot of DC breakdown voltage Vt1 _(DC) in volts as afunction of lateral base-collector spacing dimension D in micrometers,for different numbers of serially cascaded (stacked) ESD transistors 40with base-collector spacing dimension D from zone Z1 of FIG. 5. Thedesignation “ESD-Z1” is used to identify transistors 40 of FIG. 4 usedin stacked clamp 65 and 66, indicating that they individually have theVt1 _(DC) versus D characteristics illustrated by trace 61 of FIG. 8 forsingle ESD transistor 25, 40 with D chosen from zone Z1 of FIG. 5. Trace61 for single type ESD-Z1 transistor shows a sensitivity of Vt1 _(DC) tospacing dimension D of about 25 volts per micrometer, which issubstantially linear over the range from about 1.3 to about 2.4micrometers of base-collector spacing dimension D. Accordingly, toobtain a desired value of Vt1 _(DC) (and the closely related value ofVt1 _(AC)) one builds single transistor 25, 40 with the correspondingabscissa value of spacing dimension D for the desired value of Vt1 _(DC)shown on the ordinate. Trace 61 for single type ESD-Z1 device with slope(ΔVt1 _(DC)/ΔD) of about 25 volts per micrometer corresponds to centralregion Z1 of FIG. 5.

Trace 62 of FIG. 8 illustrates the variation of DC breakdown voltage Vt1_(DC) as a function of spacing dimension D of 2-stack clamp 65 of FIG. 6wherein each transistor 40-1, 40-2 is of type ESD-Z1 illustrated in FIG.4 and individually having the properties illustrated by trace 61 of FIG.8, with D chosen from central region Z1 of FIG. 5. Trace 63 of FIG. 8illustrates the variation of DC breakdown voltage Vt1 _(DC) as afunction of spacing dimension D of 3-stack clamp 66 of FIG. 7 whereineach transistor 40-1, 40-2, 40-3 is of type ESD-Z1 illustrated in FIG. 4and individually having the properties illustrated by trace 61 of FIG. 8with D chosen from central region Z1 of FIG. 5. In general,(Vt1)_(2-STACK)˜(Vt1)₄₀₋₁+(Vt1)₄₀₋₂, and(Vt1)_(3-STACK)˜(Vt1)₄₀₋₁+(Vt1)₄₀₋₂+(Vt1)₄₀₋₃, etc. Serially cascading(stacking) individual ESD-Z1 transistors 40 having the responseillustrated in trace 61 of FIG. 8 enables higher trigger voltages(Vt1)_(STACK) to be obtained than would otherwise be possible withsingle ESD-Z1 device 40. It will also be noted that slope (ΔVt1_(DC)/ΔD) of the Vt1 versus D traces increases about in directproportion to the number of transistors 40 in the stack. For example,single transistor ESD-Z1 (see trace 61 of FIG, 8) has (ΔVt1_(DC)/ΔD))_(SINGLE-Z1) of about 25 volts per micrometers, 2-stack clamp65 (see trace 62 of FIG. 8) has (ΔVt1 _(DC)/ΔD)_(2-STACK-Z1) of about 50volts per micrometer and 3-stack clamp 66 (see trace 63 of FIG. 8) has(ΔVt1 _(DC)/ΔD)_(3-STACK-Z1) of about 75 volts per micrometer over therange of spacing dimension D illustrated in FIG. 8. Thus, by cascadingindividual ESD-Z1 transistors 40, a wide range of Vt1 _(DC) (and alsoVt1 _(AC)) values can be obtained. This is very useful.

The variation in Vt1 _(DC) (and Vt1 _(AC)) across the die or wafer forsuch stacked arrangements can be estimated from the slope (ΔVt1_(DC)/ΔD) of traces 61-63 of FIG. 8. Suppose for example, that theeffective variation of spacing dimension D across the die or wafer orfrom wafer to wafer (e.g., because of differences in azimuthalorientation) corresponds to about ΔD micrometers. Accordingly, whereonly a single type ESD-Z1 device is used (e.g., trace 61 of FIG. 8),then using the values for the slopes of trace 61 provided above, thevariation ΔVt1 _(DC) (and ΔVt1 _(AC)) across the die or wafer for asingle device would be about (ΔVt1)_(SINGLE-Z1)˜ΔD*25 volts. This canamount to about (ΔVt1)_(SINGLE-Z1)˜3 to 4 volts. While such variationmay be tolerable (although undesirable), the situation becomes muchworse when such devices are cascaded since, as noted above, thesensitivity ΔVt1 to spacing dimension variations ΔD increasesapproximately in direct proportion to the number of ESD-Z1 devices 40 inthe ESD stack. For example, in 2-stack clamp 65, the expected variationis about (ΔVt1)_(2-STACK-Z1)˜ΔD*50 volts, and for 3-stack clamp 66 thevariation is about (ΔVt1)_(3-STACK-Z1)˜ΔD*75 volts, which can amount toabout (ΔVt1)_(2-STACK-Z1)˜6 to 8 volts and (ΔVt1)_(3-STACK-Z1)˜9 to 12volts for the same ΔD variation as described above. This is highlyundesirable, especially since the use of stacked ESD devices oftencannot be avoided in order to obtain Vt1 values in the desired voltagerange. Accordingly, it is important to provide a means and method bywhich the excess sensitivity of trigger voltage Vt1 in stacked ESDdevices to variations in spacing dimension D can be minimized Statedanother way, it is important to be able to reduce the slope (ΔVt1/ΔD) ofthe Vt1 versus spacing dimension D characteristics of multi-devicestacks of ESD transistors.

FIG. 9 shows a simplified cross-sectional view of illustrative 2-StackESD clamp 99 implemented in semiconductor substrate 72 according to afurther embodiment of the invention. 2-stack ESD clamp 99 comprises twoseries coupled ESD transistors 70, 70′ of the type illustrated in FIG.4. The same reference numbers are used in FIG. 9 to identify the variousregions of transistors 70, 70′ as are used in FIG. 4, the discussion ofwhich can be referred to for further details. A prime (′) has been addedto the corresponding regions of transistor 70′. (Where a thirdtransistor is serially coupled to transistors 70, 70′ of FIG. 9 to forma 3-stack clamp such as is described schematically later in FIGS. 12-15,the various regions of such third transistor 70″ can be identified bythe same reference numbers as in devices 70, 70′ but with a double prime(″) added thereto. To avoid unduly cluttering the drawings, suchconfigurations are omitted in the device cross-sections.) Transistor 70has lateral Lbe spacing 50 between doped regions 77, 78 and transistor70′ has lateral Lbe spacing 50′ between doped regions 77′, 78′. LateralLbe spacings 50, 50′ may be the same or different for transistors 70,70′, as is discussed more fully later.

Transistors 70, 70′ have different values of spacing dimensions D, withtransistor 70 in this example having spacing dimension D_(Z2) betweenboundaries 752, 929 corresponding to Zone Z2 of FIG. 5 and transistor70′ having spacing dimension D_(Z1) between boundaries 752′, 929′corresponding to Zone Z1 of FIG. 5. By way of example and not intendedto be limiting, ESD transistor 70′ is shown at the right of FIG. 9 withcollector contact conductor 82′ coupled to terminal 821 leading in thecase of a 2-stack to I/O terminal 22, or in the case of a 3-stack orhigher to the next device in the stack. Transistor 70 is shown at theleft in FIG. 9 with emitter-base contact conductor 83 coupled to GNDterminal 23. Collector contact conductor 82 of transistor 70 is coupledto base-emitter contact 83′ of transistor 70′. NBL regions 73, 73′ areelectrically isolated by central and peripheral deep trench isolation(DTI) walls 792, 792′ so that transistors 70, 70′ can be coupled inseries to form 2-stack 99 (or a portion of a 3-stack or higher). It doesnot matter which of transistors 70, 70′ has spacing dimension D_(Z1) andwhich has spacing dimension D_(Z2), and such spacing may be interchanged(i.e., commuted). This commutability applies to all of the D_(Z1),D_(Z2), D_(Z3), etc., sequence variations discussed in connection withFIGS. 10-15 following. In a preferred embodiment, N WELLS 76, 76′ areannular in plan view shape and laterally surround interior elements 74,75, 77, 78, etc., and 74′, 75′, 77′, 78′, etc., respectively. In otherembodiments, N WELLS 76, 76′ need not be annular in plan view shape, butcan terminate at dielectric isolation walls lying in planes above andbelow the plane of FIG. 10. Either arrangement may be used.

FIGS. 10-15 show simplified schematic diagrams of serially cascadedstacks 100-105 of ESD transistors 70 (and 70′, 70″, etc.), wherein FIGS.10-12 shows 2-stack clamps 100, 101, 102 and FIGS. 12-14 show 3-stackclamps 103, 104, 105 according to various still further embodiments ofthe present invention. The various ESD transistors incorporated instacks 100-105 have spacing dimensions D corresponding to zones Z1, Z2and/or Z3 in these examples. The convention is adopted of identifyingthe type of ESD transistor according to which range of D values is beingused therein. For example, transistor type ESD-Z1 indicates that spacingdimension D of such transistor is chosen from zone Z1 of FIG. 5,transistor type ESD-Z2 indicates that spacing dimension D of suchtransistor is chosen from zone Z2 of FIG. 5, and transistor type ESD-Z3indicates that spacing dimension D of such transistor is chosen fromzone Z3 of FIG. 5. 2-stack clamp 100 of FIG. 10 shows first transistor70-1 of type ESD Z2 with a first lead coupled to terminal 23 of circuit20 of FIG. 1 and a second lead serially coupled to a first lead ofsecond transistor 70-2 of type ESD-Z1 whose second lead is coupled toterminal 22 of circuit 20 of FIG. 1. The cross-sectional viewillustrated in FIG. 9 corresponds to 2-stack clamp 100. 2-stack clamp101 of FIG. 11 shows first transistor 70-3 of type ESD Z1 with a firstlead coupled to terminal 23 of circuit 20 of FIG. 1 and a second leadserially coupled to a first lead of second transistor 70-4 of typeESD-Z3 whose second lead is coupled to terminal 22 of circuit 20 ofFIG. 1. 2-stack clamp 102 of FIG. 12 shows first transistor 70-5 of typeESD Z2 (or type ESD-Z3, not shown) with a first lead coupled to terminal23 of circuit 20 of FIG. 1 and a second lead serially coupled to a firstlead of second transistor 70-6 of type ESD-Z3 (or type ESD-Z2, notshown) whose second lead is coupled to terminal 22 of circuit 20 ofFIG. 1. The serial order of transistors 70-1, 70-2 and 70-3, 70-4 and70-5, 70-6 stacks clamps 100-102 is not significant and they may beinterchanged (commuted). Stated another way, according to variousembodiments of the invention, serially coupled 2-stacks of transistors40-i and 40-j desirably use any combination of type ESD-Z1, ESD-Z2, andESD-Z3 clamp transistors except two serially coupled type ESD-Z1transistors.

3-stack clamp 103 of FIG. 13 shows first transistor 70-7 of type ESD-Z2with a first lead coupled to terminal 23 of circuit 20 of FIG. 1 and asecond lead serially coupled to a first lead of second transistor 70-8of type ESD-Z1 whose second lead is coupled to a first lead of thirdtransistor 70-9 of type ESD-Z2 whose second lead is coupled to terminal22 of circuit 20 of FIG. 1. 3-stack clamp 104 of FIG. 14 shows firsttransistor 70-10 of type ESD-Z1 with a first lead coupled to terminal 23of circuit 20 of FIG. 1 and a second lead serially coupled to a firstlead of second transistor 70-11 of type ESD-Z3 whose second lead iscoupled to a first lead of third transistor 70-12 of type ESD-Z3 whosesecond lead is coupled to terminal 22 of circuit 20 of FIG. 1. 3-stackclamp 105 of FIG. 15 shows first transistor 70-13 of type ESD-Z2 with afirst lead coupled to terminal 23 of circuit 20 of FIG. 1 and a secondlead coupled to a first lead of second transistor 70-14 of type ESD-Z1whose second lead is serially coupled to a first lead of thirdtransistor 70-15 of type ESD-Z3 whose second lead is coupled to terminal22 of circuit 20 of FIG. 1. As noted above in connection with 2-stackclamps 100-102, the order of serially coupled transistors 70-7, 70-8,70-9 and 70-10, 70-11, 70-12 and 70-13, 70-14, 70-15 in 3-stack clamps103-105 does no the corresponding spacing dimensions D_(Z1), D_(Z2),D_(Z3) may be commuted in such transistors. According to variousembodiments of the invention, serially coupled 3-stacks of transistors40-i, 40-j and 40-k desirably use any combination of type ESD-Z1,ESD-Z2, and ESD-Z3 clamp transistors except two or more serially coupledtype ESD-Z1 transistors. Stated another way, no more than one ESDtransistor with spacing dimension D corresponding to zone Z1 should beserially coupled with at least one ESD transistor with spacing dimensionD corresponding to zones Z2 and/or Z3. Stated still another way, the ESDtransistor stack(s) of various embodiments of the present invention cancomprise zero or one ESD transistor(s) with spacing dimension D_(Z1)from zone Z1 serially coupled to one or more ESD transistors withspacing dimension D_(Z2) and/or D_(Z3) from zone Z2 and/or zone Z3 orboth zones Z2 and/or Z3. The order of such ESD transistors in the stackis not important. While only 2-stack and 3-stack ESD clamps areillustrated in FIGS. 10-15, persons of skill in the art will understandthat any number of ESD devices 70-1 . . . 70-N can be stacked to obtainthe desired trigger voltage Vt1 _(STACK).

FIG. 16 shows simplified plot 87 of DC breakdown voltage Vt1 _(DC) involts as a function of lateral base-collector spacing dimension D inmicrometers for different 2-stack clamps of cascaded ESD transistors,comparing the results for a 2-stack clamp of transistors 40 to theresults with 2-stack clamps of transistors 70 according to embodimentsof the present invention. Trace 87-1 (“2-stack Z1+Z1”) shows thevariation in Vt1 _(DC) with base-collector spacing dimension D for a2-stack clamp having type ESD-Z1 transistors 40 with spacing dimension Dchosen from zone Z1. Trace 87-1 of FIG. 16 and trace 62 of FIG. 8 showsubstantially the same data. Trace 87-2 and trace 87-3 show thevariation in Vt1 _(DC) with base-collector spacing dimension D for2-stack clamps of transistors 70 according to various embodiments of thepresent invention. Trace 87-2, identified as “2-stack Z1+Z2”,corresponds to a 2-stack having first ESD transistor (“ESD-Z1”) withspacing dimension D_(Z1) chosen from zone Z1 of FIG. 5 serially coupledto second transistor (“ESD-Z2”) having spacing dimension D_(Z2) chosenfrom zone Z2 of FIG. 5. Trace 87-3, identified as “2-stack Z1+Z3”,corresponds to a 2-stack having first ESD transistor (“ESD-Z1”) withspacing dimension D_(Z1) chosen from zone Z1 of FIG. 5 serially coupledto second transistor (“ESD-Z3”) having spacing dimension D_(Z3) chosenfrom zone Z3 of FIG. 5. Trace 87-2 corresponds to 2-stack clamp 99 ofFIGS. 9 and 2-stack clamp 100 of FIG. 10, and trace 87-3 corresponds to2-stack clamp 101 of FIG. 11 according to embodiments of the presentinvention. Traces 87-2, 87-3 have slopes (ΔVt1/ΔD) that are similar andabout half that of trace 87-1 of 2-stack clamp 65 of FIG. 6. Since thevariation ΔVt1 across the die or wafer is approximately directlyproportional to slope (ΔVt1/ΔD) of the Vt1 versus D plots, 2-stacksclamps 100, 101 of the present invention have about half the sensitivityto spacing variation ΔD as 2-stack 65. The variation ΔVt1 across the dieor wafer of further 2-stack clamp 102 and 3-stack clamps 103-105 offurther embodiments of the present invention is similarly proportionalto the differences in slope (ΔVt1/ΔD), and in the case of 3-stack clamps103-105 have about one-third the ΔVt1 sensitivity to unavoidable spacingvariations ΔD of 3-stack clamp 66. Thus, by choosing base-collectorspacing dimensions D of the various transistors 70 used to formmulti-transistor ESD stacks according to the rules explained above, thesensitivity of stacked ESD clamps to spacing variations ΔD can besignificantly reduced compared to multi-transistor ESD stacks thatutilize only base-collector spacing dimensions D from central zone Z1 ofFIG. 5. Further, the arrangements for obtaining such reduced sensitivityto unavoidable spacing variations ΔD do not limit the range of Vt1values than can be obtained nor significantly increase the occupieddevice area or add further manufacturing process steps. This is asignificant advance in the art and very useful for providing a widerange of ESD trigger voltages Vt1 for ICs and other devices and circuitsat no significant increase in cost.

FIG. 17 shows a simplified cross-sectional view of ESD clamp 700implemented in semiconductor substrate 72 (e.g., P), analogous to ESDclamp transistor 70 of FIG. 4, but according to a yet further embodimentof the present invention wherein a bidirectional ESD function isprovided. ESD clamp 700 comprises ESD transistor 70 (e.g., at the leftin FIG, 17) of the type described in connection with FIG. 4, combinedwith ESD transistor 70R (e.g., at the right in FIG. 17) of the same typeas ESD transistor 70 of FIG. 4 but laterally mirrored or reflected incenter plane 701. The discussion of the various regions of ESDtransistor 70 of FIG. 4 applies to FIG. 17 and the same referencenumbers are used in connection with ESD transistor 70 at the left ofFIG. 17. Analogous reference numbers are also used in connection withtransistor 70R at the right of FIG. 17 modified by the addition of “R”to indicate that they are laterally reflected or mirrored in plane 701compared to the equivalent regions in transistor 70. Thus, ESDtransistor 70 of FIG. 17 (like ESD transistor 70 of FIG. 4) has P WELLregion 75, N WELL regions 761, 762 (collectively 76), P+ region 77, N+region 78, avalanche zone 84 in portion 85 of P region 74, further Nregion 86, STI regions 79, DTI regions 792, dielectric layer 81,conductive base-emitter contact conductor 83, base-collector spacingdimension D between boundaries 752, 929, and Lbe spacing 50 between(e.g., P+) doped region 77 and (e.g., N+) doped region 78. Contactconductor 83 is conveniently coupled to GND or common terminal 23 (seeFIG. 1). ESD transistor 70R has equivalent regions 75R, 761R, 762R, 76R,77R, 78R, 84R, 85R, 79R 792R, 81R, 83R, base-collector spacing dimensionDR between boundaries 752R and 929R, and Lbe spacing 50R between (e.g.,P+) region 77R and (e.g., N+) region 78R, in mirror configuration to thecorresponding regions in transistor 70, Lbe spacings 50, 50R may be thesame or different for transistors 70, 70R. Substrate 72 and NBL 73 arecommon. N WELL collector region 762 of transistor 70 and N WELLcollector region 762R of transistor 70R are merged around center plane701. Base-emitter contact conductor 83R is coupled to terminal 821Rwhich is coupled to the next ESD device in the ESD stack. Becausecollector regions 672 and 672R are merged, conductor 82 of device 70 ofFIG. 4 is not needed in the arrangement of device 700 of FIG. 17. Forthe same reason, N+ collector contact region 80 (and 80R) of device 70of FIG. 4 is also conveniently omitted, but may be included in otherembodiments.

Bi-directional ESD devices 700 are incorporated in ESD stack clamps100-105 of FIGS. 10-15 in substantially the same manner as ESD clamps70, wherein a first device 700-1 will have D and DR chosen, for example,from zone Z1 and a second, serially coupled device 700-2 will have D andDR chosen from Z2 or Z3 or a combination thereof depending upon thenumber of devices 700 being included in the stack. In furtherembodiments, device 700-1 and/or 700-2, etc., may be chosen from zonesZ2 and/or Z3 and no transistor from zone Z1 included in the stack. Allsuch variations are useful. The discussion associated with FIGS. 4-16should be referred to for further details and variations according tostill further embodiments of the invention wherein, for example, devicesof type 700 may be substituted for any of devices 70-1 through 70-15,etc., in clamps 100-105. If base-collector spacing dimensions D and DRare substantially the same within device 700, then Vt1 will besubstantially the same for both polarities of voltage applied acrossterminals 22, 23 of stacks 100-105 of FIGS. 10-15 comprising seriallycoupled devices 700-1, 700-2, etc., of FIG. 17. However, in otherembodiments, wherein it is desired that Vt1 be different for differentpolarities, then different values may be chosen for spacing dimensions Dand DR within one or more of devices 700-1, 700-2, etc.

FIGS. 18-22 and 23-28 are simplified cross-sectional views of ESD clamp99 of FIG. 9 during various stages 118-122 and 123-128 of manufactureillustrating resulting structures 218-222 and 223-228, according tostill yet further embodiments of the present invention and showingadditional detail. Manufacturing stages 122-1 and 122-2 of FIGS. 22-1and 22-2 are discussed later. As noted in connection with FIG. 9, ESDclamp 99 is formed by laterally combining and serially couplingtransistors 70, 70′ illustrated individually in FIG. 4 but havingdifferent base-collector spacing dimensions D. In FIG. 9, the samereference numbers as in FIG. 4 are used to identify the various regionsof leftward device 70, and the same reference numbers with a prime (′)added are used to identify analogous regions of rightward device 70′.This same convention is followed in FIGS. 18-28. Doping levels and/ordoping doses are included in the discussion of FIGS. 18-28 by way ofdescribing a preferred embodiment and not for limitation. Ionimplantation is a preferred method of doing various regions withindevice 99 but is not intended to be limiting and other doping methodswell known in the art may also be used. Photo resist is a suitablemasking material for use with ion implant doping and is presumed in whatfollows unless otherwise specifically noted, but is not intended to belimiting. Persons of skill in the art will understand that other typesof masking layers or materials well known in the art may also be useddepending on the dopant and doping method desired to be used. Similarly,the manufacturing process is illustrated for silicon semiconductor, byway of example and not limitation. Those of skill in the art willunderstand that substantial modification of the choice of semiconductormaterial, dopants, doping methods, doping levels and/or doses anddimensions of various regions within device 99 may be made dependingupon the particular device characteristics desired and that thosepresented herein are not intended to be limiting. Further, it will beunderstood that while FIGS. 18-22 and 23-28 illustrate the variousstages used to form device 99 of FIG. 9, that substantially the samestages are used to form devices 40, 70 of FIG. 4 and device 700 of FIG.17 with appropriate mask changes that will be understood by those ofskill in the art.

Referring now to manufacturing stage 118 of FIG. 18, initialsemiconductor substrate 72 i (e.g., P type) is provided wherein lowerportion 721 is preferably boron doped at about 5E18 cm⁻³ and upperportion (e.g., EPI-1) 722 with upper surface 724 is preferably borondoped at about 2E15 cm⁻³. Upper portion 722 is preferably formed byepitaxial deposition referred to, for example, as “EPI-1”. Thickness 723of EPI-1 layer 722 is conveniently in the range of about 7 to 8micrometers, but larger or smaller thicknesses may also be used.Structure 218 results.

Referring now to manufacturing stage 119 of FIG. 19, mask 90 havingclosed portions 901 and open portions 902, 903 is applied over surface724 of structure 218. Implant A is provided through open portions 902,903 to form, respectively NBL regions 73 and 73′ in substrate 72 i. Forsilicon SC, antimony is a suitable dopant for providing NBL layers 73,73′. A peak doping density of about 1E19 cm⁻³ and depth 731 in the rangeof about 1 to 2 micrometers below surface 724 is preferred but otherdopants, concentrations and depths may also be used. Structure 219results. Referring now to manufacturing stage 120 of FIG. 20, mask 90 isremoved and second epitaxial layer 740 (e.g., “EPI-2”) of thickness 741of about 3 to 4 micrometers is formed on surface 724 over substrate 72 iwith NBL regions 73, 73′. EPI-2 layer 740 is conveniently boron doped toa doping density of about 2E15 cm⁻³, but larger or smaller doping levelsand thicknesses may also be used. EPI-2 layer 740 has upper surface 71and provides P regions 74. 74′ of device 99. Structure 220 results,hereafter referred to as substrate 72.

Referring now to manufacturing stage 121 of FIG. 21, conventionalshallow trench isolation (STI) regions 79, 79′ are formed at the desiredlocations in surface 71 of structure 220 (substrate 72) and deep trenchisolation (DTI) regions 792 are formed in substrate 72 extending, forexample, to initial substrate region 721, using means well known in theart. DTI regions 792 are typically formed of grown or deposited siliconoxide or a combination thereof and may in various embodiments have apoly-silicon core (not shown) according to the available manufacturingprocesses. Either arrangement is useful. Peripheral DTI regions 792,792′ at the left and right, respectively, of FIG. 21 and following,electrically isolate devices 70, 70′ from other devices on the die.Centrally located DTI regions 792, 792′ electrically isolate transistors70, 70′ from each other so that they may be placed electrically inseries (stacked). Structure 221 results. Referring now to manufacturingstage 122 of FIG. 22, mask 91 having closed portions 911 and openings912, 913 is applied over surface 71 of EPI-2 layer 740 of substrate 72.P-type Implant B of, for example, boron is provided through openings912, 913 of mask 91 to form P WELLs 75, 75′ having lateral boundaries752, 752′, respectively, at the indicated locations, P WELL 75 beingassociated with leftward device 70 and P WELL 75′ being associated withrightward device 70′. P WELLs 75, 75′ preferably have depth 751 fromsurface 71 of about 30-60 percent of EPI-2 thickness 741 and peak dopantconcentration in the range of about 1E17 cm⁻³ to 1E18 cm⁻³ with about4E17 cm⁻³ to 8E17 cm⁻³ being preferred at about 0.5 to 1.0 micrometersbelow surface 71, but larger and smaller depths and dopant densities andother dopants may also be used. Structure 222 results.

Referring now to manufacturing stage 123 of FIG. 23, mask 91 is removedand replaced with mask 92 having closed portions 921 and openings 922,923, 924 and 925. N-type Implant C is provided through openings 922,923, 924 and 925 to form N WELL regions 926, 927 associated withtransistor 70 and N WELL regions 926′, 927′ associated with transistor70′, wherein lateral boundary 929 is associated with region 927 andboundary 929′ is associated with region 927′, with spacing dimensionD_(Z2) between boundaries 752, 929 and spacing dimension D_(Z1) betweenboundaries 752′, 929′. Phosphorous is a suitable dopant for such N WELLregions. A peak concentration in the range of about 1E17 cm⁻³ to 1E18cm⁻³ is useful with about 4E17 cm⁻³ to 8E17 cm⁻³ being preferred,located about 1 to 1.5 micrometers beneath surface 71 and total depth928 about 30-60 percent larger than depth 751 of P WELLs 75, 75′ butless than thickness 741 of EPI-2 layer 740, but other dopants and dopingdensities and depths may also be used. Structure 223 results. Therelative lateral location of opening 912 in mask 91 for forming P WELLregion 75 in manufacturing stage 122 versus the location of opening 923of mask 92 for forming N WELL region 927 in manufacturing stage 123determines spacing dimension D_(Z2) of device 70 of ESD clamp 99. Therelative lateral location of opening 913 in mask 91 for forming P WELLregion 75′ in manufacturing stage 122 versus the location of opening 925of mask 92 for forming N WELL region 927′ in manufacturing stage 123determines spacing dimension D_(Z1) of device 70′ of ESD clamp 99.Persons of skill in the art will understand based on the descriptionherein that other combinations of spacings D_(Z1), D_(Z2), and D_(Z3)may be obtained by adjusting the relative lateral locations of such PWELL and N WELL openings in masks 91, 92.

Referring now to manufacturing stage 124 of FIG. 24, mask 92 is removedand replaced with mask 93 having closed portions 931 and openings 932,933, 934 and 935. N-type Implant D is provided through openings 932,933, 934 and 935 to form N WELL regions 936, 937 associated withtransistor 70 and N WELL regions 936′, 937′ associated with transistor70′. In a preferred embodiment, openings 932, 934 of mask 93 aresubstantially coincident with openings 922, 924 of mask 92, but in otherembodiments may differ. Again, in a preferred embodiment, openings 933and 935 of mask 93 are narrower and lie within openings 923 and 925 ofmask 92, but may have other extents in further embodiments. The purposeof mask 93 with openings 932, 933, 934 and 935 and Implant D is toextend N WELL regions 926, 927, 926′, 927′ formed in manufacturing stage123 to form N WELL regions 936, 937, 936′, 937′ that make Ohmic contactwith NBLs 73, 73′. Phosphorous is a suitable dopant for Implant D toform such N WELL regions. A first peak concentration in the range ofabout 5E17 cm⁻³ to 2E18 cm⁻³ at a first depth below surface 71 in therange of about 0.5 to 1 micrometers and a second peak concentration inthe range of about 4E17 cm⁻³ to 1E18 cm⁻³ at a second depth belowsurface 71 in the range of about 1.5 to 2 micrometers and a total depthsufficient to Ohmically couple N WELL regions 936, 937, 936′, 937′ toNBL 73, 73′ is preferred, but other dopants and doping densities anddepths may also be used. Structure 224 results. N WELLS 936, 937, 936′and 937′ correspond to N WELLS 761, 762, 761′ and 762′ of device 99 ofFIG. 9 and are identified as such hereafter. The portions of N WELLs927, 927′ that extend laterally beyond N WELLs 937, 937′ correspond tofurther N regions 86, 86′ of FIG. 9, and are identified as suchhereafter. Manufacturing stages 123, 124 may be performed in eitherorder and may be performed before or after manufacturing stage 122.

Referring now to manufacturing stage 125 of FIG. 25, mask layer 93 isremoved and “silicide block” layer 94 applied over surface 71 andpatterned to provide silicide block regions 941 wherever it is desiredto prevent reaction between a silicide forming conductor (to bedeposited later) and exposed portions of semiconductor surface 71, andopenings 942 between silicide block regions 941 where it is desired forthe subsequently deposited conductor to form silicides ensuring goodOhmic contact to the underlying portions of SC surface 71. In apreferred embodiment, silicide block layer 94 is also conveniently usedto define Lbe spacing 50, 50′, 50R of FIGS. 4, 9, 17, 29 and 30. Forexample, openings 9421, 9422 of silicide block layer 94 are separated bylateral distance 501 and openings 9424, 9425 are separated by lateraldistance 501′, substantially corresponding in lateral size to desiredLbe spacings 50, 50′ or 50, 5OR of FIGS. 4, 9, 17, 29 and 30. In apreferred embodiment, silicide block layer 94 preferably comprises afirst layer of silicon oxide of about 10-20 nanometers thicknessoverlying surface 71 followed by a second layer of silicon nitride ofabout 40-80 nanometers thickness overlying the first layer, but othermaterials and thicknesses may also be used in other embodiments and theterms “silicide block layer” and “silicide block, regions” are intendedto include such variations. Structure 225 results. While provision ofsilicide block regions 941 is desirable, they may be omitted in yetfurther embodiments and lateral separations 50, 50′, 50R, etc., may bedetermined by the lateral spacing of openings in masks 95 and 96. Insubsequent FIGS. 26-28, silicide block regions 941 are indicated bydashed lines, but are ignored in FIGS. 4, 9, 17, 29 and 30 since theybecome incorporated in surface dielectric layer 81, 81′ applied inmanufacturing stage 128.

Referring now to manufacturing stage 126 of FIG. 26, mask 95 is appliedover surface 71 and silicide block regions 941, mask 95 having closedportions 951 and openings 952, 953, 954, 955 corresponding to thedesired locations of (e.g., N+) doped Ohmic collector contacts andemitter regions to be formed using implant E. Implant E (e.g., N type)is provided through openings 952, 953, 954, 955 wherein (e.g., N+)emitter region 78 is formed through opening 952, (e.g., N+) Ohmiccollector contact region 80 is formed through opening 953, (e.g., N+)doped emitter region 78′ is formed through opening 954, and (e.g., N+)doped Ohmic collector contact region 80′ is formed through opening 955.In order to minimize alignment issues, it is preferred that openings952, 953, 954, 955 of mask 95 are chosen to be slightly wider thanunderlying openings 9421-9424 (see FIG. 25) in silicide block layer 94.In other embodiments where silicide block layer 94 is omitted, openings952, 953, 954, 955 may correspond to the desired final width andlocation of the doped regions being formed by Implant E. The dopant,energy and dose of Implant E are desirably selected so as to providerelatively shallow highly doped (e.g., N+) regions 78, 78′, 80, 80′, asfor example, employing arsenic dopant, with a peak concentration ofabout 1E20 cm⁻³ or greater and depth of about 0.2-0.3 micrometers, butother dopants and depths may also be used. Structure 226 results. In apreferred embodiment in which block layer 94 is used, the implant energyand block layer thickness are chosen so that block layer 94 can act as amask to Implant E. In this way, lateral mask separations 501, 501′ ofmanufacturing stage 125 of FIG. 25 substantially determine Lbe spacing50, 50′, etc., as will become apparent from manufacturing stage 127.

Referring now to manufacturing stage 127 of FIG. 27, mask 95 is removedand mask 96 applied with closed portions 961 and openings 962, 963corresponding to the desired locations of (e.g., P+) doped Ohmic basecontact regions 77, 77′ to be formed using implant F. Implant F (e.g., Ptype) is provided through openings 962, 963 wherein (e.g., P+) dopedOhmic base contact region 77 is formed through opening 962 and (e.g.,P+) doped Ohmic base contact region 77′ is formed through opening 963.In order to minimize alignment issues, it is preferred that openings962, 963 of mask 96 are chosen to be slightly wider than underlyingopenings 9421-9424 (see FIG. 25) in silicide block layer 94. In otherembodiments where silicide block layer 94 is omitted, openings 962, 963may correspond to the desired final width and location of the dopedregions being formed by Implant F. The dopant, energy and dose ofImplant F are selected so as to provide relatively shallow highly doped(e.g., P+) base contact regions, as for example, employing boron dopantwith a peak concentration of about 1E20 cm⁻³ or greater and depth ofabout 0.2-0.3 micrometers, but other dopants and depths may also beused. Structure 227 results. In a preferred embodiment in which blocklayer 94 is used, the implant energy and block layer thickness arechosen so that block layer 94 acts as a mask to Implant F. In this way,lateral mask separation 501, 501′ of manufacturing stage 125 of FIG. 25substantially determines Lbe spacing 50, 50′, etc., as previouslyindicated.

Referring now to manufacturing stage 128 of FIG. 28, mask 96 is removed,dielectric layer 81, 81′ is applied and patterned to expose thoseportions of doped contact or emitter regions 77, 78, 80, 77′, 78′, 80′where Ohmic electrical contact is desired to be made and conductors 82,82′, 83, 83′ applied therein and coupled to terminals 23, 821, all usingmeans and methods well known in the art. In a preferred embodiment inwhich blocking layer 94 is used, the contact openings provided indielectric layer 81, 81′ may be made slightly wider than the openings inblocking layer 94 in order to minimize alignment issues, since blockinglayer 94 is still present on surface 71 and insures that Ohmic contactis made by conductor 82, 82′ to doped SC regions 77, 77′, 78, 78′ formedusing the same blocking layer as part of the implant masks for ImplantsE and F. This is indicated in FIG. 28 but ignored in FIGS. 4, 9, 17, 29and 30 to avoid unduly cluttering such drawings. Structure 228 results.ESD clamp 99 is substantially complete. While manufacturing stages118-128 have illustrated how 2-stack 100 of FIGS. 9 and 10 may beformed, persons of skill in the art will understand based on thedescription herein how other stacks 101-105 and even single ESD devicesmay be fabricated in an analogous manner using substantially the samemanufacturing stages as those described herein with appropriate maskchanges within the competence of those of skill in the art. For example,by adjusting the relative lateral spacings and numbers of openings 912,913, etc., of manufacturing stage 122 for producing (e.g., P type) WELLS75, 75′, etc., versus the location and numbers of openings 923, 925,etc., of manufacturing stage 123 for producing (e.g., N type) WELLS 927,927′ (and resulting (e.g., N type) regions 86, 86′), the numbers ofserially stacked transistors 70-i+70-j+70-k, etc. (where, i, j, k . . .equal 1, 2, 3, . . . ), and their associated spacing dimensions D_(Z1),D_(Z2), D_(Z3), etc., may be varied to yield any desired stackcombination to provide the desired trigger voltage Vt1 _(STACK), asdiscussed in connection with FIGS. 10-15. It will also be appreciatedbased on the description herein that stacks of dual-polarity devices 700of FIG. 17 may be provided using the illustrated manufacturing stageswith appropriate alterations of the various mask openings to yield thedevice regions and interconnections identified in connection with FIG.17. It should be further understood that while the manufacturing stagesequence illustrated in FIGS. 18-28 is preferred, this is intended forconvenience of explanation of a preferred embodiment and not intended tobe limiting, and in additional embodiments the sequence of variousmanufacturing stages may be interchanged. For example and not intendedto be limiting, while formation of BL 73, 73′ is shown as occurringafter formation of EPI-1 layer 722 and prior to formation of EPI-2 layer740, in other embodiments, it may be formed after formation of EPI-2layer 740 or at other stages of the manufacturing process. Additionally,while portions of substrate 72 including region 74, 74′ are preferablyformed epitaxially, in still additional embodiments, such regions may bepart of an initial substrate in which devices 70, 70′, 99, 700, etc.,are subsequently formed and one or both epitaxial deposition steps(e.g., EPI-1 and/or EPI-2) may be omitted. Still further, while in thepreferred embodiment, (e.g., N type) WELLs 761, 762, 761′, 762′ areformed by multiple masking and implant steps, in other embodiments, suchmultiple masking and implant steps may be combined provided that theserially coupled ESD transistors in the ESD stack have differentbase-collector spacing dimensions D, with none or no more than onespacing dimension D chosen from zone Z1 and others chosen from zones Z2and/or Z3 of FIG. 5, with the number depending upon the total number ofstacked transistors being employed to achieve the desired value of Vt1_(STACK). In further embodiments, DTI regions 792, 792′ may be omittedand lateral isolation of ESD clamp transistors 70, 70′, etc., beprovided by means of (e.g., N type) WELLS 761, 762, etc., orequivalents. Either arrangement is useful.

The foregoing has explained how various embodiments illustrated abovemake it possible to provide ESD devices of a wide range of triggervoltages Vt1 without increasing their sensitivity to (e.g., angular andlot to lot) manufacturing variations, as have been experienced withprior art ESD devices. However, other ESD properties are also ofinterest. For example, it is important that the device damage-onsetcurrent threshold (e.g., It2 of FIG. 3) of the ESD device be larger thanthe anticipated worst case ESD current. Thus, it is important to be ableto increase It2 for a given ESD device. FIG. 29 is a simplifiedcross-sectional view of ESD clamps 56, analogous to ESD clamps 40, 70,99, 700 previously described and suitable for use in the circuits ofFIGS. 1-2, but according to another embodiment of the present invention.FIG. 29 is somewhat simplified compared to FIGS. 4, 9 and 17. Forexample, layer 81 and conductors 82, 83 are omitted in order to avoidunduly cluttering the drawing. Where appropriate, the same referencenumbers are used in FIG. 29 to identify analogous regions and referenceshould be made to the discussion of FIGS. 4, 9, 17 and 18-28 for variousvalues of depth, thickness, dopant density, etc. For example, baseportion 52 of FIG. 29 is also identified as region 75-1 to indicate, byway of example and not intended to be limiting, that it can be made(other than geometry) in the same manner as region 75 of FIGS. 4 and 22.

It has been found that parasitic base-emitter resistance Rbe 29 (seealso FIG. 2) can significantly impact damage-onset current threshold It2of ESD devices. By increasing Rbe, It2 can be increased. Rbe can beincreased in several ways, for example: (i) by increasing Lbe 50, (ii)by decreasing the doping density or increasing the sheet resistance ofthe portion of the base proximate emitter 78 and base contact region 77(for example and not intended to be limiting in region 51 of FIGS. 29and 30), or (iii) increasing both Lbe and the sheet resistance of thatportion of the base. The increase in Rbe achieved in these ways, canincrease both the DC and AC (transient) values of It2, withoutsignificant adverse effect on other ESD device properties. This is verydesirable.

Tables I and II below illustrate the comparative behavior of singlestack ESD devices of the type illustrated in FIG. 29. In both Tables Iand II, devices with modified Rbe are obtained by either increasing Lbe(second data line) or increasing the base sheet resistance (third dataline), or both (fourth data line). The results of making such changesare compared to otherwise substantially identical but unmodified (i.e.,“baseline”) ESD devices whose properties are shown in the first dataline of each table.

TABLE I COMPARATIVE TRANSIENT RESPONSE OF 1-STACK ESD DEVICES Base SheetLbe Resistance Vt1 Vsb It2 % It2 % AREA (micro-meters) (ohms/sq) (volts)(volts) (amps) INCREASE INCREASE 1.75 800 23.6 14.6 6.63 BaselineBaseline 4.0 800 23.6 13.7 7.00 5.6 4.9 1.75 2800 23.6 13.3 7.20 8.6 06.0 2800 23.6 11.9 7.70 11.6 9.3

TABLE II COMPARATIVE DC RESPONSE OF 1-STACK ESD DEVICES Base Sheet It2Lbe (micro- Resistance Vt1 Vsb (amps) % It2 % AREA meters) (ohms/sq)(volts) (volts) (see note) INCREASE INCREASE 1.75 800 22 8.1 0.65Baseline Baseline 4.0 800 22 6.3 >0.77 >18.2 4.9 1.75 2800 226.1 >0.77 >18.2 0 6.0 2800 22 5.8 >0.77 >18.2 9.3 (NOTE: The maximumobservable DC current here was limited by the tester to 0.77 amps)

It is apparent that increasing Rbe, for example: (i) by increasing Lbe,or (ii) by increasing the resistivity (e.g., sheet resistance) of thebase region proximate the emitter and base contact (e.g., in region 51of FIG. 29) or (iii) by increasing both, provides improved (i.e.,larger) It2. Thus, the ability of the ESD device to absorb an ESDcurrent is improved and the ESD device is more robust. Where this isaccomplished by increasing Lbe, this produces an increase in device areacompared to a baseline (unmodified) device of the same base resistivity.However, in both the AC and DC tests of devices with increased Lbe, thepercent improvement in It2 is several times the percent area increase.This is favorable. When Lbe is kept unchanged, and the base resistivity(e.g., sheet resistance) increased, no increase in area results eventhough a significant improvement in It2 is obtained. This is verydesirable.

In those cases in TABLES I and II where Lbe was changed compared to thebaseline device, both portions 51 and 52 of the base in FIG. 29 had thesame doping and the same configuration as described in connection withWELL region 75 of FIGS. 4 and 22. The doping on either side of gap D wasunchanged as between the baseline devices and the devices with modifiedLbe. The Vt1 values for the modified devices were the same as thebaseline device, indicating that increasing Lbe in this way did notsubstantially alter the breakdown mechanism. In those cases in TABLES Iand II where the base sheet resistance was changed, this change was madein region 51 (see FIG. 29) proximate base contact region 77 and emitter78, but not in region 52 proximate boundary 752 of avalanche region 84in base portion 85. The doping in region 52 corresponds to thatdescribed for WELL 75 (see the discussion of FIGS. 4 and 22). Leavingportion 52 (e.g., portion 75-1) of the base adjacent boundary 752unchanged from the baseline device minimizes any adverse effect on Vt1of changing the base resistivity in region 51 and the observed Vt1values are substantially the same as for the baseline device. Region 52has lateral width 53 so as to separate boundary 752 from higherresistivity portion 51 in preferred embodiments.

For a given base doping, the minimum Rbe is determined primarily by theminimum achievable value of Lbe, usually set by the minimum achievablefeature spacing of the particular fabrication process being used. It hasbeen found that Rbe values of at least about 1 Ohms and up to about 800Ohms are useful. Rbe values in the range of about 2 to 240 Ohms are moreconvenient for increasing It2, which values of Rbe correspond to Lbespacings of about 2.5 to about 32 times a minimum Lbe spacing of, forexample, about 0.7 micrometers. It2 improves as Lbe (and Rbe) isincreased for values of Lbe of about 11-12 micrometers (e.g., Rbe˜120Ohms) and becomes saturated for Lbe values of about 23 micrometers andlarger. Accordingly, Lbe distance 50 is usefully in the range of about0.7 to 23 micrometers, more desirably in the range of about 1.0 to 15micrometers, and preferably about at least 10-12 micrometers, but largerand smaller values can also be used. Varying Lbe can also be used toadjust the properties of ESD devices 56, 58 to suit particularsituations. For example, for a given doping level in base portion 51,75, smaller Lbe provides higher Vh and compact size but can lower It2.Larger Lbe provides higher It2 but can lower Vh. Thus, adjusting Lbe aswell as base doping allows different combinations of ESD deviceproperties to be selected to suit particular applications. This isuseful.

Base portion 51 encompassing base contact region 77 and emitter 78 hasdoping usefully in the range of about 1E16 to 1E18 cm⁻³ moreconveniently in the range of about 4E16 to 4E17 cm⁻³, and preferably inthe range of about 4E16 to 8E16 cm⁻³, but higher and lower values canalso be used. Base portion 52 with boundary 752 has doping usefully inthe range of about 1E17 to 1E18 cm⁻³ and preferably in the range ofabout 4E17 to 8E17 cm⁻³, but higher and lower values can also be used.Stated another way, base portion 51 has sheet resistance that isusefully at least about twice that of base portion 52, preferably in therange of about 2 to 20 times that of base portion 52. Stated stillanother way, the dopant concentration in base portion 51 proximate basecontact 77, 77′, 77R and emitter 78, 78′, 78R versus the doping of baseportion 52 proximate boundary 752, is usefully less than the doping ofbase portion 52, more conveniently less than about one half the dopingof base portion 52 and as small as about one twentieth the doping ofbase portion 52. Stated yet another way, the doping of base portion 52is at least twice that of base portion 51 and preferably as large astwenty times the doping of base portion 51. Depth 54 of base portion 51below surface 71 can be larger or smaller than the depth of base portion52, and is usefully in a range of about 0.5 to 2 micrometers.

That data in Tables I and II indicate that damage-onset thresholdcurrent It2 may be improved by adjusting the intrinsic base resistanceRbe in any of several ways, for example such as those described above.This beneficial effect is also obtained in multi-stack ESD devices. Ingeneral, it is preferred to similarly adjust Rbe of all devices in thestack, but in other embodiments, only some of the devices in the stackmay be modified for this purpose, depending upon the design goals forthe ESD stack. Either arrangement is useful.

When it is desired to increase Rbe merely by increasing Lbe, no changesin doping profiles are needed compared to what is described inconnection with FIGS. 4, 9, 17 and 18-28. Manufacturing stages 118-128of FIGS. 18-28 can be used as is and the adjustment in Lbe (andtherefore in Rbe) made merely by adjusting spacing 501, 501′ betweenopenings 9421, 9422 and 9424, 9425, respectively, of blocking layer 941of manufacturing stage 125 of FIG. 25, the number of such openingsdepending upon the number of devices in the ESD stack. Spacings 501,501′ determine Lbe distance 50, 50′, 50R of FIGS. 27-28, 4, 9, 17, 29and 30. By increasing distance 501, 501′, distance Lbe can becorrespondingly increased and the desired Lbe ranges (e.g., such asthose illustrated above) readily achieved.

When it is desired to increase Rbe by decreasing the doping of, forexample, base portion 51 of FIG. 29, thereby increasing the sheetresistance of base portion 51 of FIG. 29, several differentmanufacturing arrangements may be used. In one embodiment, base portion51 of FIG. 29 may be formed by using the relatively lower backgrounddoping of EPI-2 region 74, 740 (see FIG. 20), which may be adjusted toprovide the desired ESD properties. This is accomplished by limitingmask opening 912, 913 of FIG. 22 so that it does not extend over whatwill become base portion 51 of FIG. 29. In this embodiment, mask opening912, 913 of FIG. 22 is laterally narrowed to correspond merely toportion 52 (75-1) of FIG. 29, that is, (e.g., P type) WELL 75 of FIG. 22encompasses only portion 52 (75-1) of FIG. 29. FIG. 30 is a simplifiedcross-sectional view of resulting ESD clamp 58 analogous to ESD clamps40, 70, 99, 700, 56 previously described and suitable for use in thecircuits of FIGS. 1-2, but according to the embodiment described herewhere base portion 51 of FIG. 29 (shown dashed in FIG. 30) is formedusing the background doping of EPI-2 layer 74, 740 of FIG. 20 and WELL75 of FIG. 22 corresponds just to portion 52 of FIG. 30, that is, toWELL region 75-1 in FIG. 30. Persons of skill in the art will understandthat the doping of EPI-2 region 74, 740 of FIG. 20 may be adjusted withthis embodiment in mind so as to provide the desired values of Rbe.

In another embodiment, where it is desired that region 51 of FIG. 29have a doping different than that of EPI-2 layer 74, 740 of FIG. 20, forexample, intermediate between the doping of Epi-2 layer 74, 740 and thatillustrated above for base portion 52 (75-1). To accomplish this,manufacturing stage 122 of FIG. 22 may be altered as illustrated by wayof example in manufacturing stages 122-1 and 122-2 of FIGS. 22-1 and22-2. FIGS. 22-1 and 22-2 are simplified cross-sectional views of an ESDclamp of the type illustrated in FIG. 9 showing variations ofmanufacturing stage 122 of FIG. 22, according to additional embodimentsof the present invention and showing further detail. In a preferredembodiment, manufacturing stage 122 of FIG. 22 comprising Implant B isreplaced by manufacturing stage 122-1 comprising Implant B1 andmanufacturing stage 122-2 comprising Implant B2. In manufacturing stage122-1 of FIG. 22-1, mask 91-1 is provided on SC surface 71, havingclosed portions 911-1 and openings 912-1 and 913-1, suitable fordefining WELL regions 51 i, 51 i′. A suffix “i” is used here to indicatethat initial base portions 51 i, 51 i′ may differ in lateral extent fromfinal base portions 51, 51′ of FIG. 29. In a preferred embodiment, WELLregion 51 i, 51 i′ may have the same lateral dimensions as WELL region75, 75′ of FIG. 22, and depth 54 may be the same as depth 751 of FIG.22, but larger or smaller lateral dimensions and depths may also beused. However, the doping desired for base WELL region 51 i, 51 i′resulting from Implant B1 of FIG. 22-1 is desirably smaller than thatfor WELL region 75 resulting from Implant B of FIG. 22. Region 51 i, 51i′ is desirably implanted to provide a doping density in the range ofabout 1E16 to 1E18 cm⁻³ and preferably in the range of about 4E16 to8E16 cm⁻³, but higher and lower doping may also be used. Structure 222-1results.

Referring now to manufacturing stage 122-2 of FIG. 22-2, mask 91-1 ofmanufacturing stage 122-1 is replaced with mask 91-2 having closedportions 911-2 and openings 912-2, 913-2. Implant B2 is provided throughopenings 912-2, 913-2, so as to form base portions 52, 52′ having dopingdensity usefully in the range of about 1E17 to 1E18 cm⁻³, preferably inthe range of about 4E17 to 8E17 cm⁻³, but higher and lower doping mayalso be used. Structure 222-2 results. In a preferred embodiment, otherthan geometry, Implant B2 can be similar to Implant B of FIG. 22 forproviding region 75. Accordingly, in FIGS. 29-30, portion 52 is alsoidentified as region 75-1.

In manufacturing step 122-1 of FIG. 22-1, mask openings 912-1 and 913-1providing base portions 51 i, 51 i′ are shown as being wide enough tosignificantly overlap at least part of the location of mask openings912-2 and 913-2 of manufacturing stage 122-2 of FIG. 22-2 providing baseportions 52, 52′. While this is preferred since it simplifies alignment,it is not necessary and in still further embodiments, such overlap maybe eliminated. In this configuration mask openings 912-1 and 913-1 ofmanufacturing stage 122-1 of FIG. 22-1 may be narrowed to eliminate someor all overlap with subsequently formed base portions 52, 52′. Then baseportions 51 i, 51 i correspond substantially to base portions 51, 51′substantially side-by-side with base portions 52, 52′ rather thansignificantly overlapping base portions 52, 52′. Either arrangement isuseful.

Having completed manufacturing stage 122-2, structure 222-2 is subjectedto manufacturing stages 123-128 of FIGS. 23-28 keeping in mind thepresence of base portions 51, 52, thereby yielding the structureillustrated in FIGS. 29 and 30 where base portion 51 has a differentdoping density (e.g., lighter), than the doping density of base portion52. In a preferred embodiment, base portion 51 has doping densityintermediate between the doping density of base portions 74, 85 formedfrom EPI-2 layer 74, 740 of FIG. 20 and the doping density of baseportion 52. In this way, the resistance Rbe of transistor 56 isincreased since the sheet resistance of base portion 51 is increasedrelative to base portion 52, thereby improving damage-onset thresholdcurrent It2. Because the doping density of base portion 52 issubstantially unchanged, the improvement in It2 is achieved withoutsignificantly altering Vt1. Accordingly, a more robust ESD device havingsubstantially the same threshold voltage Vt1 can be obtained in bothsingle stage and multi-stage devices. This is very desirable. Anadvantage of modifying Rbe by changing the sheet resistance of baseportion 51 is that no increase in ESD device area results. This is alsodesirable. While it is desirable to provide base portion 52 (e.g., ofhigher doping density) proximate boundary 752, in other embodiments suchhigher doping density base portion 52 may be omitted. In such cases, thedoping density of WELL region 75 of FIGS. 4, 9, 17, etc., can beadjusted to provide the desired increase in Rbe and spacing D modifiedto achieve the desired Vt1 value in either single or multi-stackdevices. Either arrangement is useful.

Persons of skill in the art will understand based on the teachingsherein that Rbe (and therefore It2) can be increased by varying eitherthe sheet resistance of base portion 51 (e.g., as illustrated inconnection with FIGS. 29-30 and FIGS. 22-1, 22-2) or by increasing thesheet resistance of WELL portion 75 as a whole (e.g., as illustrated inFIGS. 4, 9, 17, etc.,) or by varying Lbe (e.g., as discussed inconnection with FIGS. 25, 29, 22-1 and 22-2) or by a combination of sucharrangements. Thus, the embodiments of the present invention illustratedherein provide a high degree of flexibility in improving ESDdamage-onset threshold current It2 to obtain more robust ESD devices.This flexibility is very useful for tailoring ESD devices to suitspecific applications and is a particular advantage of the presentinvention.

According to a first embodiment, there is provided an electrostaticdischarge (ESD) clamp having a predetermined threshold voltage Vt1,comprising, a bipolar transistor (56, 58) having a first surface (71)and underlying the first surface (71) having a base (51, 52, 74, 75, 85)of a first conductivity type, a base contact (77) of the firstconductivity type extending into the base (51, 52, 74, 75, 85) from thefirst surface (71), an emitter (78) of a second, opposite, conductivitytype extending into the base (51, 52, 74, 75, 85) from the first surface(71) and laterally separated from the base contact (77) at the firstsurface (71) by a distance Lbe, and a collector (86, 762) proximate thebase (51, 52, 74, 75, 85), wherein the base (51, 52, 74, 75, 85)comprises a first base portion (51) containing the base contact (77) andthe emitter (78) and having a first base dopant concentration, a secondbase portion (52) located laterally between the first portion (51) and afirst boundary (752) and having a second base dopant concentration, anda third base portion (85) of width D located laterally between theboundary (752) and a portion (86) of the collector (86, 762) and havinga third base dopant concentration, wherein the threshold Vt1 isdetermined at least in part by the width D, and wherein the first basedopant concentration is less than or equal the second base dopantconcentration. According to a further embodiment, Lbe is in the range ofabout 0.7 to 23 micrometers. According to a still further embodiment,Lbe is in the range of about 1 to 15 micrometers. According to a yetfurther embodiment, the first base portion (51) has a doping density inthe range of about 1E16 to 1E18 cm⁻³. According to a yet still furtherembodiment, the second base doping density is at least twice the firstbase doping density. According to another embodiment, the first baseportion (51) has a first base sheet resistance and the second baseportion (52) has a second base sheet resistance, and the first basesheet resistance is larger than the second base sheet resistance.According to still another embodiment, the first base sheet resistanceis at least twice the second base sheet resistance. According to yetanother embodiment, the first base sheet resistance is at least 2 to 20times the second base sheet resistance. According to a still yet anotherembodiment, the transistor (56, 58) has a parasitic emitter- baseresistance Rbe (29) in the range of 1≦Rbe≦800 Ohms. According to a yetstill another embodiment, the transistor (70) has a distance D=D1, andthe ESD clamp (21) comprises a second transistor (70′) analogous to thefirst transistor (70) but having a distance D′=D2 different than D1.

According to a second embodiment, there is provided a method for formingan ESD clamp (99, 100-105), comprising, providing a semiconductorsubstrate (72) of a first conductivity type and having an upper surface(71), forming at least one transistor (70) having a first well region(75, 51, 52) of a first conductivity type and first doping densityextending into the substrate (72) from the first surface (71), the firstwell region (75, 51, 52) having a first lateral edge (752), forming inthe at least one transistor (70) another well region (927) of a secondopposite conductivity type extending into the substrate from the firstsurface (71), the another well region (927) forming part of a collector(27) of the transistor (70) and having another lateral edge (929)laterally separated from the first lateral edge (752) by a distance Dadapted to determine in part a threshold voltage Vt1 of the ESD clamp(99, 100-105), forming in the first well region (52, 75) a base contact(77) of the first conductivity type and an emitter (78) of the secondconductivity type separated by a lateral distance Lbe, and wherein thefirst doping density and the lateral distance Lbe are selected toprovide a parasitic base-emitter resistance Rbe (29) in the range of1≦Rbe≦800 Ohms According to a further embodiment, the first well region(51, 52, 75) has a first base portion (51) of a first base portiondoping density and containing the base contact (77) and the emitter(78), and the first well region (51, 51, 75) also has a second baseportion (52) of a second base portion doping density, located betweenthe first base portion (51) and the first lateral edge (752), and thesecond base portion doping density is greater than the first baseportion doping density. According to a yet further embodiment, the firstbase portion (51) has a first sheet resistance and the second baseportion (52) has a second sheet resistance and the first sheetresistance is at least about twice the second sheet resistance.According to still yet further embodiment, the emitter (78) and the basecontact region (77) are separated by a lateral distance Lbe in the rangeof about 0.7≦Lbe≦23 micrometers. According to a yet still furtherembodiment, the first base doping density is in the range of about 1E16cm⁻³ to 1E18 cm⁻³.

According to a third embodiment, there is provided an electrostaticdischarge (ESD) protection clamp (21) for protecting a circuit core(24), comprising a bipolar transistor (56, 58) comprising a base region(74, 51, 52, 85) with a base contact (77) therein, an emitter (78)located in the base region (74, 51, 52, 85) spaced a lateral distanceLbe from the base contact (77), and a collector (80, 86, 762) proximatethe base region (74, 51, 52, 85), wherein the base contact (77) and theemitter (78) are electrically coupled together and to a first terminal(23) of the clamp (21) and the collector (80, 86, 762) is electricallycoupled to a second terminal (22) of the clamp (21) and the first (23)and second (22) terminals are adapted to be coupled across the circuitcore (24), wherein the base region (74, 51, 52, 85) comprises a firstportion (51) having a first doping density, the first portion (51)comprising the base contact (77) and the emitter (78) separated by alateral distance Lbe, wherein the base region (74, 51, 52, 85) furthercomprises a second portion (52) having a second doping density, thesecond portion (52) having a lateral boundary (752) facing toward aportion (86, 762) of the collector (80, 86, 762), wherein the secondportion (52) separates the first portion (51) from the lateral boundary(752), and wherein the first doping density is less than the seconddoping density. According to a further embodiment, the first dopingdensity is less than or equal to one-half the second doping density.According to a still further embodiment, the second doping density is inthe range of about 1E17 to 1E18 cm⁻³. According to a yet furtherembodiment, the base region (74, 51, 52, 85) has a parasiticbase-emitter resistance Rbe (29) in the range of about 1 to 800 OhmsAccording to a yet further embodiment, Rbe (29) is in the range of about2 to 240 Ohms

While at least one exemplary embodiment and method of fabrication hasbeen presented in the foregoing detailed description of the invention,it should be appreciated that a vast number of variations exist. Itshould also be appreciated that the exemplary embodiment or exemplaryembodiments are only examples, and are not intended to limit the scope,applicability, or configuration of the invention in any way. Rather, theforegoing detailed description will provide those skilled in the artwith a convenient road map for implementing an exemplary embodiment ofthe invention, it being understood that various changes may be made inthe function and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

1. An electrostatic discharge protection (ESD) clamp having apredetermined threshold voltage Vt1, comprising: a bipolar transistorhaving a first surface and underlying the first surface having a base ofa first conductivity type, a base contact of the first conductivity typeextending into the base from the first surface, an emitter of a second,opposite, conductivity type extending into the base from the firstsurface and laterally separated from the base contact at the firstsurface by a distance Lbe, and a collector proximate the base; whereinthe base comprises a first base portion containing the base contact andthe emitter and having a first base dopant concentration, a second baseportion located laterally between the first portion and a first boundaryand having a second base dopant concentration, and a third base portionof width D located laterally between the boundary and a portion of thecollector and having a third base dopant concentration, wherein thethreshold Vt1 is determined at least in part by the width D; wherein thefirst base dopant concentration is less than the second base dopantconcentration; and wherein Lbe is in the range of about 0.7 to 23micrometers.
 2. The ESD clamp of claim 1, wherein Lbe is in the range ofabout 1 to 15 micrometers.
 3. The ESD clamp of claim 1, wherein thefirst base portion has a doping density in the range of about 1E16 to1E18 cm⁻³.
 4. The ESD clamp of claim 1, wherein the second base dopingdensity is at least twice the first base doping density.
 5. The ESDclamp of claim 1, wherein the transistor has a parasitic emitter- baseresistance Rbe in the range of 1≦Rbe≦800 Ohms.
 6. The ESD clamp of claim1, wherein the transistor has a distance D=D1, and the ESD clampcomprises a second transistor analogous to the first transistor buthaving a distance D′=D2 different than D1.
 7. The ESD clamp of claim 1,wherein the first base portion has a first base sheet resistance and thesecond base portion has a second base sheet resistance, and the firstbase sheet resistance is larger than the second base sheet resistance.8. The ESD clamp of claim 7, wherein the first base sheet resistance isat least twice the second base sheet resistance.
 9. An electrostaticdischarge protection (ESD) clamp having a predetermined thresholdvoltage Vt1, comprising: a bipolar transistor having a first surface andunderlying the first surface having a base of a first conductivity type,a base contact of the first conductivity type extending into the basefrom the first surface, an emitter of a second, opposite, conductivitytype extending into the base from the first surface and laterallyseparated from the base contact at the first surface by a distance Lbe,and a collector proximate the base; wherein the base comprises a firstbase portion containing the base contact and the emitter and having afirst base dopant concentration, a second base portion located laterallybetween the first portion and a first boundary and having a second basedopant concentration, and a third base portion of width D locatedlaterally between the boundary and a portion of the collector and havinga third base dopant concentration, wherein the threshold Vt1 isdetermined at least in part by the width D; wherein the first basedopant concentration is less than the second base dopant concentration;and wherein the first base portion has a first base sheet resistance andthe second base portion has a second base sheet resistance, and thefirst base sheet resistance is larger than the second base sheetresistance.
 10. The ESD clamp of claim 9 wherein the first base sheetresistance is at least twice the second base sheet resistance.
 11. TheESD clamp of claim 9 wherein the first base sheet resistance is at least2 to 20 times the second base sheet resistance.
 12. The ESD clamp ofclaim 9, wherein Lbe is in the range of about 0.7 to 23 micrometers. 13.The ESD clamp of claim 9, wherein the first base portion has a dopingdensity in the range of about 1E16 to 1E18 cm⁻³.
 14. The ESD clamp ofclaim 9, wherein the second base doping density is at least twice thefirst base doping density.
 15. The ESD clamp of claim 9, wherein thetransistor has a parasitic emitter-base resistance Rbe in the range of1<Rbe<800 Ohms.
 16. The ESD clamp of claim 9, wherein the transistor hasa distance D=D1, and the ESD clamp comprises a second transistoranalogous to the first transistor but having a distance D′=D2 differentthan D1.
 17. An electrostatic discharge (ESD) protection clamp forprotecting a circuit core, comprising: a bipolar transistor comprising abase region with a base contact therein, an emitter located in the baseregion spaced a lateral distance Lbe from the base contact, and acollector proximate the base region; wherein the base contact and theemitter are electrically coupled together and to a first terminal of theclamp and the collector is electrically coupled to a second terminal ofthe clamp and the first and second terminals are adapted to be coupledacross the circuit core; wherein the base region comprises a firstportion having a first doping density, the first portion comprising thebase contact and the emitter separated by a lateral distance Lbe;wherein the base region further comprises a second portion having asecond doping density, the second portion having a lateral boundaryfacing toward a portion of the collector, wherein the second portionseparates the first portion from the lateral boundary; wherein the firstdoping density is less than the second doping density; and wherein thebase region has a parasitic base-emitter resistance Rbe in the range ofabout 1 to 800 Ohms.
 18. The ESD protection clamp of claim 17, whereinthe first doping density is less than or equal to one-half the seconddoping density.
 19. The ESD protection clamp of claim 17, wherein thesecond doping density is in the range of about 1E17 to 1E18 cm⁻³. 20.The ESD protection clamp of claim 17, wherein Rbe is in the range ofabout 2 to 240 Ohms.